The present invention concerns improved nondestructive testing of a workpiece, in general, and in particular an improved scanner head and corresponding method for magnetic particle inspection of a magnetizable workpiece.
One widely known nondestructive test for discontinuities or cracks in ferromagnetic materials is magnetic particle inspection. It is known that magnetizing a ferromagnetic workpiece will cause opposing magnetic fields, that is flux leakages, to form at discontinuities in the workpieces. Application of magnetic particles, in either a wet or dry medium, to a magnetized ferromagnetic material will cause attraction of such magnetic particles in the areas of such flux leakages.
With experience, testing technicians can discern the differences between attraction of magnetic particles to cracks or the like in the workpiece versus the attraction pattern of magnetic particles to sharp edges or other intended features of the workpiece. Various methodologies relating to the general concepts of magnetic particle testing or inspection are well known to those of ordinary skill in the art of nondestructive inspection, as exemplified by the section entitled "Magnetic-Particle Inspection" at pages 44 through 74 of the Metals Handbook, 8th edition, Volume 11, "Nondestructive Inspection and Quality Control", by the American Society for Metals, 1976. Certain magnetic particle inspection techniques are also known by the term "magnafluxing".
Generally, in order to adequately test a workpiece, successive magnetization steps must be undertaken so that the article to be inspected is magnetized in at least two directions, normally approximately 45.degree. to 90.degree. from each other. Often, the two required directions of magnetization are obtained, as a practical matter by different methods of producing a magnetic field. As one example, a round shaft can be magnetized in a circular direction by physical contact with the shaft resulting in the passing of a current therethrough. Using well known magnetic field theory, such operation provides only one direction of magnetization of the round shaft. Another direction of magnetization may be obtained by placing the shaft within a coil, which would result in a longitudinally directed magnetization.
Even from the foregoing very simple-shaped workpiece example, it may be seen that considerable skilled handling and manipulation of the workpiece is required in order to effect multi-axis magnetization. As the complexity of the workpiece increases, so does the complexity and challenge of performing magnetic particle inspection on the workpiece.
In addition to the foregoing technical problems associated with physically manipulating and handling a workpiece, often requiring a highly experienced technician for successful testing operations, certain conventional testing procedures can result in damage to the workpiece. For example, whenever current is physically passed through a workpiece, electrical and physical contact with such workpiece is needed. Thus, in view of the sometimes thousands of ampere currents involved, a danger of arcing and burning the part exists. Moreover, workpiece areas adjacent to or near contact points (such as underneath a contact pad) cannot be inspected, requiring additional steps to obtain 100% coverage of the workpiece being inspected.
It is also a general aspect of conventional magnetic particle inspection procedures that current requirements for adequate magnetization increase considerably in direct relation to increasing size of the workpiece to be tested. Of course, significant increases in current levels correspondingly increases the potential for arcing and burning.
Still further, concern and experience must typically be brought to bear in determining the best (i.e., most efficient and safest) testing methodology to effect the magnetic particle inspection for a given workpiece. Whenever a coil is used for magnetization, the workpiece itself must be able to fit inside the coil. In addition, close attention must be paid to the relative closeness of the workpiece outside diameter to the coil inside diameter (i.e., the fill factor of the coil). Calculations for the required current are conventionally based on the length to diameter ratio of the workpiece, during such coil magnetizations. On occasion, pole pieces are needed to adjust this ratio into accepted or available amperage ranges. Accordingly, manipulation of the workpiece and rearrangement of the testing layout between successive magnetization steps, particularly for larger and/or odd-shaped pieces, is a very time consuming process.
In addition to magnetization with coils, magnetic yokes are known for providing magnetization in two directions by successively placing the yoke in different positions over the area of the workpiece to be tested. Normally, such yokes require physical contact with the part, which leads to obvious difficulties and disadvantages in connection with the testing of complex shaped parts. Accordingly, thorough testing of a given workpiece for either large or complex shaped parts is also time consuming whenever magnetic yokes are utilized.
Another magnetization methodology which can provide twin directional magnetization with subsequent operations involves use of a prod. Like yokes, prods can be applied in different positions to the workpiece of interest, resulting in magnetic field direction manipulation. Prods all normally involve electrical and physical contact with the part. Since use of a prod in essence results in small area contact with the workpiece, there is a considerably increased danger of burning the workpiece surface due to arcing.
Still further magnetic particle inspection methodologies are known, but commonly share operational difficulties or drawbacks when adapting same to variety in sizes and shapes of a workpiece. Also, larger parts to be tested commonly preclude use of small portable power supply-driven testing units due to their relatively larger power consumption requirements.
In all of the foregoing instances where physical setups and interconnections with the workpiece being tested must be determined and altered during successive steps, testing is relatively slow and tedious. Obviously, the more difficult it is to test a given piece, the more likely the possibility of improper or inadequate (e.g., incomplete) testing of such given workpiece. It may often be the case that complex shaped pieces which are relatively more difficult to test may in fact have a relatively greater need for testing due to additional processing steps in which they may have been involved in order to achieve their complex shape, since it is well known that processing steps such as cutting, grinding, or the like can introduce discontinuities, cracks, or other imperfections which are sought to be detected by use of magnetic particle inspections.
U.S. Pat. Nos. 4,694,247, issued to Meili et al., and 3,763,423, issued to Forster, are cited as examples of magnetic particle testing technology. The general teachings of such references as relates to known magnetic particle testing theories and the like are incorporated herein by reference, for additional general background of nondestructive magnetic particle testing.
More particularly, Meili et al. discloses a magnetic yoke type magnetization device which to some extent avoids direct physical contact with the workpiece. Elongated cylindrical or polygonal-shaped parts, i.e. long shafts, may be tested by rotating same about their axis, while also advancing same longitudinally along a cushion of dry magnetic material applied to the surface of the workpiece in the area of an adjacent magnetic yoke leg. While there is a degree of non-contact as between the magnetization device and the workpiece, the types of workpieces which may be treated with the Meili et al. device are very strictly limited. In others words, the Meili et al. teachings are not useful for general magnetization purposes, particularly for shapes other than long shafts. Also, the Meili et al. structure is rather involved, which does not suggest practical portability thereof.
Forster similarly relates to magnetization of an elongated workpiece being conveyed along a path by first and second test stations. The test stations incorporate a pair of orthogonally related magnetizing cores and respective energizing windings. The platelike core members each have a respective energizable winding, with an edge of each plate-like core being disposed closely adjacent passing surfaces of the workpiece. Thus, magnetization is effected with a narrower side edge of a plate-like core, rendering the Forster structure efficient primarily only with the elongated billet-type workpieces illustrated, such as having rectangular, circular, or hexagonal crosssections.